Ophthal. Visibility oftransparent objects in the eye by

Brit. _. Ophthal. (I97I) 55, 517

Visibility of transparent objects in theeye by retroillumination

NICHOLAS BROWN

Moorfields Eye Hospital, City Road, London

Retroillumination is performed at the slit-lamp with the binocular microscope focusedupon the object which is illuminated from behind by the slit-lamp beam reflected off somestructure posterior to the object (Vogt, I930). Objects in the cornea and lens are observedin this way and the reflectors used are the iris, the lens, or the fundus. The use of theophthalmoscope for retroillumination of the cornea is also described (Chandler, I945).

Objects are visible because they interfere with the emergent light from the eye by ob-struction, refraction, respersion (Berliner, 1943), and reflection. Transparent objectsare visible by retroillumination when the refractive index of the object is above or belowthat of the surrounding medium. The property mainly responsible is refraction, butreflection also plays some part. The observed appearance changes with the directionof the retroillumination and with the refractive nature of the object. From the appearanceof an object under varying retroillumination conditions, conclusions can be drawn aboutthe refractive nature of the object. Although a variety of combinations of shapes andrefractive indices are possible in the object, these can be resolved into only two observableeffects:(i) The object acting as a diverging refractor (lens or cylinder).

(2) The object acting as a converging refractor (lens or cylinder).Clinical examples are shown in macrophotographs made by the technique previously

described (Brown, I970). Photographs are shown for comparison of a diverging lens(plano-concave) and of a converging lens (plano-convex) (Fig. 2). A model has beenconstructed to resemble objects found in the eye. The model has two spherical objectsimbedded in a clear medium (gelatin): one object of low refractive index (air) which actsas a diverging lens, and one of high refractive index (liquid paraffin) which acts as aconverging lens. Photographs are shown of the model under varying conditions ofillumination (Figs 3, 4, and 5).

Methods of retroilluminationMARGINAL RETROILLUMINATION (Graves, I924)

This is produced by illuminating half the field behind the object, with the object placedin front of the conjunction between light and dark areas. This can be achieved by usingthe edge of the decentred slit beam, or by using a natural boundary such as the pupilmargin.The distribution of light within the object is seen to take one of two forms:

(i) Unreversed: the distribution is the same as that of the background.(2) Reversed: the distribution is opposite to that of the background.

Received for publication February 23, 1971Address for reprints: Moorfielcls Eye Hospital, Gity Road, Londoni, E.C.1

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The unreversed effect is produced by an object acting as a diverging refractor and thereversed effect by a converging refractor, provided that its focal length is shorter than thedistance between the object and the area of retroillumination. The optical explanationof the effect is shown in Fig. IA.

FIG. I Optics of retroillumination2 3

Converginglens

Diverginglens

Highrefractive

index

Lowrefractive

index

FI G. 2 Lenses. The diverging lens shows the unreversed effect. The converging lens shows the reversed effect

FIG. 3 Model. Marginal retroillumination. The low refractive index object shows the unreversed effect.The high refractive index object shows the reversed effect

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Retroillumination of the eye

4 5

Lowrefractive

index

Highrefractive

index

FIG. 4 Model. Indirect retroillumination. The low refractive index object shows refracted and reflectedlight on one side. The high refractive index object shows reflected and refracted light on opposite sides

FIG. 5 Model. Direct retroillumination. Both objects show dark edges

In the photograph of the lenses (Fig. 2) the unreversed effect is seen in the diverginglens and the reversed effect in the converging lens. In the model (Fig. 3), the unreversedeffect is seen in the object of low refractive index and the reversed effect in the object ofhigh refractive index.

Clinical examples of spherical objects showing the unreversed effect are shown in Figs6 and 7 and the reversed effect in Figs 8 and 9. Cylindrical objects showing the unreversedeffect are shown in Figs Io and I I and the reversed effect in Figs I2 and I3.Vogt (I930) recognized that cystic objects reversed light, and he used as an example

a photograph of rain drops upon a window. Graves (I924) was able to separate theappearances into unreversed and reversed effects and suggested that this was due to therefractive nature of the object.

INDIRECT RETROILLUMINATION (Berliner, I943)This is produced by displacing the retroillumination to one side of the object, so that theobject is observed against a dark background. When this is done the unreversed orreversed effects become limited to crescents in spherical objects and to the marginal regionin cylindrical objects. These effects may be seen alone, and also in conjunction withsurface reflections (described below) as is seen in the photographs of the model (Fig. 4).The optics of indirect retroillumination are shown in Fig. IB.

Clinical examples showing crescents are seen with the unreversed effect in Figs 6 and I4,and with the reversed effect in Fig. 8. A cylindrical object showing the reversed effectlimited to the marginal region is shown in Fig. 13.

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(6)

F IG. 6 Intraepithelial cysts. AIarginal and indirect retroillumination unreversed effect and reflections

F I.G 7 Net-like dystrophy. Marginal retroillumination. Unreversed effect

Surface reflections occur with both spherical and cylindrical objects observed by indirectretroillumination. The reflection occurs on the same side as the illumination, so that thereflection contributes to the crescent of the unreversed effect, but is on the opposite side tothe reversed effect. The converging type of spherical object therefore shows two brightcrescents, one on each side. The optics of reflections are shown in Fig. IB. With thelow refractive index object, this is by total internal reflection withii the denser mediu.,and with the high refractive index object it is a surface reflection off the object.

Reflections are seen in the photograph of the model (Fig. 4), and in clinical examples ofspherical objects (Figs 6 and 8), and of a cylindrical object (Fig. 13).

DIRECT RETROILLUMINATION

The object is illuminated by an evenly lit background directly behind it. The objectis then distinguished from the background by a dark edge which is present for both con-

verging and diverging objects. The luminosity of the central parts of the object appearsimilar to the background illumination. In practice it is not easy to produce a perfectlyevenly lit background, so that the dark edge effect is usually combined with the unreversedor reversed effects.



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Retroillumnination of the eye

FIG. 9 Epithelial bulla Marglnal

retrolllurnination. Rezversed effect

FIG. 8 Intraepithelial cysts. Marginaland indirect retroillumination. Reversedeffect and reflections

The dark edge is accounted for by rays being unable to pass through the peripheryof an object, acting as a thick lens to reach the observer (Fig. IC).The dark edge is shown by the spherical objects in the model (Fig. 5); the clinical

example in which it is best seen is Fig. I5.

DiscussionThe effects seen in transparent objects are resolvable into:(i) Objects acting as a diverging refractor.(2) Objects acting as a converging refractor.The ways in which these can be recognized have been described above.The shape of the object can be confidently determined in some cases with additional

clues from the appearance by focal illumination and from the disturbance of the tearfilm and of the specular reflex when present. When these clues are not available theshape of the object remains speculative.The cornea is a substance of relatively high refractive index I.376 (Matthiessen, I891)

compared to the aqueous of index I.336 (Duke-Elder, I938). The corneal epitheliumhas a higher refractive index than the stroma of I.4I6 (Fischer, I927).The following types of object have refractive properties.

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Nicholas Brown

FIG. IO Finger print lines. Mar-ginal retroillumination. Unreversedeffect

."ij..v. ...:

J:, ::.

FIG. II Stromal blood vessels. Marginal retro-illumination. Unreversed effect

FIG. 14 Lens cyst. Indirect retroillu-mination. Unreversed effectFIG. I2 Stromal microfilaria.

Marginal retroillumination.Reversed effect

FIG. 13 Descemet's splits.Marginal and indirect retroillu-mination

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Retroillumination of the eye

FIG. I 5 Intraepithelial cysts. Directretroillumination. Dark edges

DIVERGING

N~~-s/~~ cornealepithelium

W~~~~~~~~~~~~~~~~~~~~~~IX stro ma

Descemet'smembrane

CONVERGINGsolid cysi

7

objects of low objects of highrefractive index refractive index

0X//XX/ /Y///X//Xj

F I G. i6 Shape of objects visibleby retroillumination

Object interfering with a surfaceObjects which project from the anterior or posterior surfaces of the cornea into the air oraqueous respectively act as converging refractors since they project into regions of lowerrefractive index. Objects of this type are visible by focal illumination and disturb thespecular reflex and also the tear film when anterior.The shapes which these objects may take are shown in Fig. I6A. Clinical examples are

epithelial bullae (Fig. 9), endothelial colloid bodies, and the so-called Descemet's splits(Fig. 13), which project into the aqueous.

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Object projecting into a tissue of different refractive indexThis type of object can occur wherever two adjacent tissues have different refractiveindices, in particular at the junction of the corneal epithelium with Bowman's membrane.The objects act as diverging or converging refractors, depending upon whether theyproject into a zone of higher or lower refractive index. Clues to their shape are usuallynot available from other methods of examination.The shapes which these objects may take are shown diagrammatically in Fig. I6B.

Clinical examples are the fingerprint line (Fig. io) and net-like dystrophy (Fig. 7), whichboth behave as diverging refractors.

Objects whollv within a tissue of different refractive indexThis type of object is seen within the corneal epithelium, in the corneal stroma, and in thelens. When the refractive index of the object is lower than that of the surroundingtissue it acts as a diverging refractor, and when the refractive index of the object is higherit acts as a converging refractor.The shape of this type of object is shown diagramatically in Fig. i6c and is imitated

by the model. Clinical examples are intraepithelial cysts (Figs 6, 8, and I5), and bloodvessels and microfilariae in the corneal stroma (Figs ii and I2), and also the lens cyst(Fig. 14).

If the object of this type is large enough to disturb the surface it then acts as an objectinterfering with a surface such as the corneal bulla (Fig. 9).

I wish to thank Prof. Barrie R. Jones for giving me access to his patients and Mr. T. R. Tarrant of theInstitute of Ophthalmology who prepared the diagrams.

References

BERLINER, M. L. (I943) "Biomicroscopy of the Eye", vol. I, p. 79. Hoeber, New YorkBROWN, N. (1970) Brit. J-. Ophthal., 54, 697CHANDLER, P. A. (945) Amer. J. Ophthal., 28, 355

DUKE-ELDER, S. (I938) "Textbook of Ophthalmology", vol. I, p. 744. Kimpton, LondonFISCHER (I927) Ber. dtsch. ophthal. Ges., 46, 429GRAVES, B. (I924) Brit. J. Ophthal., 8, 502

MATTHIESSEN, L. (i 89 I) "Die neueren Fortschritte in unserer Kenntnis von dem optischen Baue desAuges der Wirbeltiere". Voss, Hamburg

VOGT, A. (1930) "Lehrbuch und Atlas der Spaltlampenmikroskopie des lebenden Auges, 2nd ed.Erster Teil: Technik und Methodik Hornhaut und Vorderkammer", p. 2I. Springer, Berlin

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Ophthal. Visibility oftransparent objects in the eye by